3, 7, 15, 39, 63, 127, 255, 511

Question:
3, 7, 15, 39, 63, 127, 255, 511

[A].

7

[B].

15

[C].

39

[D].

63

Answer: Option C

Explanation:

Go on multiplying 2 and adding 1 to get the next number.

So, 39 is wrong.